U.S. patent number 10,211,209 [Application Number 15/933,897] was granted by the patent office on 2019-02-19 for method of operating semiconductor memory device with floating body transistor using silicon controlled rectifier principle.
This patent grant is currently assigned to Zeno Semiconductor, Inc.. The grantee listed for this patent is Zeno Semiconductor, Inc.. Invention is credited to Yuniarto Widjaja.
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United States Patent |
10,211,209 |
Widjaja |
February 19, 2019 |
Method of operating semiconductor memory device with floating body
transistor using silicon controlled rectifier principle
Abstract
Methods of operating semiconductor memory devices with floating
body transistors, using a silicon controlled rectifier principle
are provided, as are semiconductor memory devices for performing
such operations. A method of maintaining the data state of a
semiconductor dynamic random access memory cell is provided,
wherein the memory cell comprises a substrate being made of a
material having a first conductivity type selected from p-type
conductivity type and n-type conductivity type; a first region
having a second conductivity type selected from the p-type and
n-type conductivity types, the second conductivity type being
different from the first conductivity type; a second region having
the second conductivity type, the second region being spaced apart
from the first region; a buried layer in the substrate below the
first and second regions, spaced apart from the first and second
regions and having the second conductivity type; a body region
formed between the first and second regions and the buried layer,
the body region having the first conductivity type; and a gate
positioned between the first and second regions and adjacent the
body region. The memory cell is configured to store a first data
state which corresponds to a first charge in the body region in a
first configuration, and a second data state which corresponds to a
second charge in the body region in a second configuration. The
method includes: providing the memory cell storing one of the first
and second data states; and applying a positive voltage to a
substrate terminal connected to the substrate beneath the buried
layer, wherein when the body region is in the first state, the body
region turns on a silicon controlled rectifier device of the cell
and current flows through the device to maintain configuration of
the memory cell in the first memory state, and wherein when the
memory cell is in the second state, the body region does not turn
on the silicon controlled rectifier device, current does not flow,
and a blocking operation results, causing the body to maintain the
second memory state.
Inventors: |
Widjaja; Yuniarto (San Jose,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Zeno Semiconductor, Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
Zeno Semiconductor, Inc.
(Sunnyvale, CA)
|
Family
ID: |
41652828 |
Appl.
No.: |
15/933,897 |
Filed: |
March 23, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180219013 A1 |
Aug 2, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15675523 |
Aug 11, 2017 |
9960166 |
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15290996 |
Sep 12, 2017 |
9761589 |
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14956103 |
Nov 1, 2016 |
9484082 |
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14444109 |
Jan 5, 2016 |
9230965 |
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14023246 |
Sep 16, 2014 |
8837247 |
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13244916 |
Oct 15, 2013 |
8559257 |
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12533661 |
Dec 13, 2011 |
8077536 |
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61086170 |
Aug 5, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
27/10802 (20130101); G11C 11/4096 (20130101); G11C
11/406 (20130101); H01L 29/7841 (20130101); G11C
11/404 (20130101); G11C 11/4076 (20130101); G11C
2211/4016 (20130101) |
Current International
Class: |
G11C
7/00 (20060101); H01L 29/78 (20060101); G11C
11/4076 (20060101); G11C 11/404 (20060101); H01L
27/108 (20060101); G11C 11/4096 (20060101); G11C
11/406 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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|
Primary Examiner: Le; Toan K
Attorney, Agent or Firm: Law Office of Alan W. Cannon
Parent Case Text
This application is a continuation application of co-pending
application Ser. No. 15/675,523, filed Aug. 11, 2017, which is a
continuation application of application Ser. No. 15/290,996, filed
Oct. 11, 2016, now U.S. Pat. No. 9,761,589, which issued on Sep.
12, 2017, which is a continuation application of application Ser.
No. 14/956,103, filed Dec. 1, 2015, now U.S. Pat. No. 9,484,082,
which issued on Nov. 1, 2016, which is a continuation application
of application Ser. No. 14/444,109, filed Jul. 28, 2014, now U.S.
Pat. No. 9,230,965, which issued on Jan. 5, 2016, which is a
continuation application of application Ser. No. 14/023,246, filed
Sep. 10, 2013, now U.S. Pat. No. 8,837,247, which issued on Sep.
16, 2014, which is a continuation application of application Ser.
No. 13/244,916, filed Sep. 26, 2011, now U.S. Pat. No. 8,559,257
which issued on Oct. 15, 2013, which is a continuation application
of application Ser. No. 12/533,661, filed Jul. 31, 2009, now U.S.
Pat. No. 8,077,536, which issued on Dec. 13, 2011, and which claims
the benefit of U.S. Provisional Application No. 61/086,170, filed
Aug. 5, 2008, which applications and patents are each hereby
incorporated herein, in their entireties, by reference thereto. We
claim priority to application Ser. Nos. 15/675,523; 15/290,996;
14/956,103; 14/444,109; 14/023,246; 13/244,916 and 12/533,661 under
35 U.S.C. Section 120 and claim priority to Application Ser. No.
61/086,170 under 35 U.S.C. Section 119.
Claims
That which is claimed is:
1. A semiconductor memory cell comprising: a first silicon
controlled rectifier device having a first region, a first floating
body region, a first buried layer region, and a first substrate
region; a second silicon controlled rectifier device having a
second region, a second floating body region, a second buried layer
region, and a second substrate region; wherein said first floating
body region is common to said second floating body region; wherein
said first buried layer region is common to said second buried
layer region; wherein said first substrate region is common to said
second substrate region; and wherein a state of said memory cell is
maintained through a positive voltage applied to said first and
second substrate regions.
2. The memory cell of claim 1, wherein said first floating body
region and said second floating body region have a first
conductivity type selected from a p-type conductivity type and an
n-type conductivity type; wherein said first region and said second
region have a second conductivity type selected from said p-type
conductivity type and said n-type conductivity type, said second
conductivity type being different from said first conductivity
type; wherein said first buried layer region and said second buried
layer region have said second conductivity type; and wherein said
first substrate region and said second substrate region have said
first conductivity type.
3. The memory cell of claim 1, wherein said first and second
floating body regions comprise side surfaces, said memory cell
further comprising insulating layers bounding said side
surfaces.
4. The memory cell of claim 1, wherein said positive voltage
applied to said first and second substrate regions is a constant
positive voltage bias.
5. The memory cell of claim 1, wherein said positive voltage
applied to said first and second substrate regions is a periodic
pulse of positive voltage.
6. The memory cell of claim 1, wherein said state of said memory
cell is stored in said first floating body region and said second
floating body region.
7. The memory cell of claim 6, wherein a maximum potential that can
be stored in said first and second floating body regions is
increased by said applying a voltage to said first and second
substrate regions, resulting in a relatively larger memory
window.
8. The memory cell of claim 1, wherein said first and second
floating body regions and said first and second regions are formed
in a fin that extends above said first and second buried layer
regions, and said first and second floating body regions are
between said first and second regions.
9. A semiconductor memory array comprising: a plurality of
semiconductor memory cells arranged in a matrix of rows and
columns, wherein each semiconductor memory cell comprises: a first
silicon controlled rectifier device having a first region, a first
floating body region, a first buried layer region, and a first
substrate region; and a second silicon controlled rectifier device
having a second region, a second floating body region, a second
buried layer region, and a second substrate region; wherein said
first floating body region is common to said second floating body
region; wherein said first buried layer region is common to said
second buried layer region; wherein said first substrate region is
common to said second substrate region; wherein a state of said
memory cell is maintained through a positive voltage applied to
said first and second substrate regions; wherein said first and
second buried layer regions are commonly connected to at least two
of said memory cells; and wherein said first and second substrate
regions are commonly connected to said at least two of said memory
cells.
10. The semiconductor memory array of claim 9, wherein said first
and second substrate regions are commonly connected to all of said
cells in said array.
11. The semiconductor memory array of claim 9, wherein said first
and second substrate regions are segmented to allow independent
control of bias applied on a selected portion of said memory
array.
12. The semiconductor memory array of claim 9, wherein said voltage
applied to said first and second substrate regions is a constant
positive voltage bias.
13. The semiconductor memory array of claim 9, wherein said voltage
applied to said first and second substrate regions is a periodic
pulse of positive voltage bias.
14. The semiconductor memory array of claim 9, wherein said state
of said memory cell is stored in said first and second floating
body regions.
15. The semiconductor memory array of claim 9, wherein said first
and second floating body regions and said first and second regions
are formed in a fin that extends above said first and second buried
layer regions, and said floating body region is between said first
and second regions.
16. An integrated circuit comprising: a semiconductor memory array
comprising a plurality of semiconductor memory cells arranged in a
matrix of rows and columns, wherein each said semiconductor memory
cell comprises: a first silicon controlled rectifier device having
a first region, a first floating body region, a first buried layer
region, and a first substrate region; a second silicon controlled
rectifier device having a second region, a second floating body
region, a second buried layer region, and a second substrate
region; and a control circuit configured to provide electrical
signals to said first and second substrate regions; wherein said
first floating body region is common to said second floating body
region; wherein said first buried layer region is common to said
second buried layer region; wherein said first substrate region is
common to said second substrate region; wherein a state of said
memory cell is maintained through a positive voltage applied to
said first and second substrate regions; wherein said first and
second buried layer regions are commonly connected to at least two
of said memory cells; and wherein said first and second substrate
regions are commonly connected to said at least two of said memory
cells.
17. The integrated circuit of claim 16, wherein said first and
second substrate regions are commonly connected to all of said
cells in said array.
18. The integrated circuit of claim 16, wherein said first and
second substrate regions are segmented to allow independent control
of bias applied on a selected portion of said memory array.
19. The integrated circuit of claim 16, wherein said voltage
applied to said first and second substrate regions is one of a
constant positive voltage bias or a periodic pulse of positive
voltage bias.
20. The integrated circuit of claim 16, wherein said first and
second floating body regions and said first and second regions are
formed in a fin that extends above said first and second buried
layer regions, and said first and second floating body regions are
between said first and second regions.
Description
FIELD OF THE INVENTION
The present invention relates to semiconductor memory technology.
More specifically, the present invention relates to dynamic random
access memory having an electrically floating body transistor.
BACKGROUND OF THE INVENTION
Semiconductor memory devices are used extensively to store data.
Dynamic Random Access Memory (DRAM) is widely used in many
applications. Conventional DRAM cells consist of a one-transistor
and one-capacitor (1T/1C) structure. As the 1T/1C memory cell
feature is being scaled, difficulties arise due to the necessity of
maintaining the capacitance values of each memory scale in the
scaled architecture.
There is a need in the art for improve DRAM memory that can better
retain capacitance values in the cells of a scaled architecture
comprising many DRAM memory cells. Because of the rapid growth in
the amounts of memory used by modern electronic devices, there is a
continuing need to provided improvement in DRAM architecture that
allow for a smaller cell size than the currently available 1T./1C
memory cell architecture.
Currently existing DRAM memory must be periodically refreshed to
maintain the viability of the data stored therein, as the stored
charges have a finite lifetime and begin to degrade after a period
of time. The charges therefore need to be refreshed to their
originally stored values. To do this, the data is first read out
and then it is written back into the DRAM. This process must be
repeated cyclically after each passage of a predetermined period of
time, and is inefficient, as it is both time consuming and energy
inefficient.
Thus, there is a need for DRAM memory that is both space efficient
and can be efficiently refreshed.
The present inventions satisfies these needs as well as providing
additional features that will become apparent upon reading the
specification below with reference to the figures.
SUMMARY OF THE INVENTION
The present invention provides methods of operating semiconductor
memory devices with floating body transistors, using a silicon
controlled rectifier principle and also provide semiconductor
memory devices for such operations.
A method of maintaining the data state of a semiconductor dynamic
random access memory cell is provided, wherein the memory cell
comprises a substrate being made of a material having a first
conductivity type selected from p-type conductivity type and n-type
conductivity type; a first region having a second conductivity type
selected from the p-type and n-type conductivity types, the second
conductivity type being different from the first conductivity type;
a second region having the second conductivity type, the second
region being spaced apart from the first region; a buried layer in
the substrate below the first and second regions, spaced apart from
the first and second regions and having the second conductivity
type; a body region formed between the first and second regions and
the buried layer, the body region having the first conductivity
type; and a gate positioned between the first and second regions
and adjacent the body region. The memory cell is configured to
store a first data state which corresponds to a first charge in the
body region in a first configuration, and a second data state which
corresponds to a second charge in the body region in a second
configuration. The method includes: providing the memory cell
storing one of the first and second data states; and applying a
positive voltage to a substrate terminal connected to the substrate
beneath the buried layer, wherein when the body region is in the
first state, the body region turns on a silicon controlled
rectifier device of the cell and current flows through the device
to maintain configuration of the memory cell in the first memory
state, and wherein when the memory cell is in the second state, the
body region does not turn on the silicon controlled rectifier
device, current does not flow, and a blocking operation results,
causing the body to maintain the second memory state.
In at least one embodiment, the memory cell includes, in addition
to the substrate terminal, a source line terminal electrically
connected to one of the first and second regions; a bit line
terminal electrically connected to the other of the first and
second regions; a word line terminal connected to the gate; and a
buried well terminal electrically connected to the buried layer;
the method further comprising: applying a substantially neutral
voltage to the bit line terminal; applying a negative voltage to
the word line terminal; and allowing the source line terminal and
the buried well terminal to float.
In at least one embodiment, the memory cell includes, in addition
to the substrate terminal, a source line terminal electrically
connected to one of the first and second regions; a bit line
terminal electrically connected to the other of the first and
second regions; a word line terminal connected to the gate; and a
buried well terminal electrically connected to the buried layer;
the method further comprising: applying a substantially neutral
voltage to the source line terminal; applying a negative voltage to
the word line terminal; and allowing the bit line terminal and the
buried well terminal to float.
A method of reading the data state of a semiconductor dynamic
random access memory cell is provided, wherein the memory cell
comprises a substrate being made of a material having a first
conductivity type selected from p-type conductivity type and n-type
conductivity type; a first region having a second conductivity type
selected from the p-type and n-type conductivity types, the second
conductivity type being different from the first conductivity type;
a second region having the second conductivity type, the second
region being spaced apart from the first region; a buried layer in
the substrate below the first and second regions, spaced apart from
the first and second regions and having the second conductivity
type; a body region formed between the first and second regions and
the buried layer, the body region having the first conductivity
type; and a gate positioned between the first and second regions
and adjacent the body region. The memory cell further comprises a
substrate terminal electrically connected to the substrate, a
source line terminal electrically connected to one of the first and
second regions, a bit line terminal electrically connected to the
other of the first and second regions, a word line terminal
connected to the gate, and a buried well terminal electrically
connected to the buried layer; wherein each memory cell is
configured to store a first data state which corresponds to a first
charge in the body region in a first configuration, and a second
data state which corresponds to a second charge in the body region
in a second configuration. The method includes: applying a positive
voltage to the substrate terminal; applying a positive voltage to
the word line terminal; applying a substantially neutral voltage to
the bit line terminal; and allowing voltage levels of the source
line terminal and the buried well terminal to float; wherein, when
the memory cell is in the first data state, a silicon controlled
rectifier device is formed by the substrate, buried well, body
region and region connected to the bit line terminal is in
low-impedance, conducting mode, and a higher cell current is
observed at the bit line terminal compared to when the memory cell
is in the second data state, as when the memory cell is in the
second data state, the silicon rectifier device is in blocking
mode.
A semiconductor memory array is provided, including: a plurality of
semiconductor dynamic random access memory cells arranged in a
matrix of rows and columns, each semiconductor dynamic random
access memory cell including: a substrate having a top surface, the
substrate being made of a material having a first conductivity type
selected from p-type conductivity type and n-type conductivity
type; a first region having a second conductivity type selected
from the p-type and n-type conductivity types, the second
conductivity type being different from the first conductivity type,
the first region being formed in the substrate and exposed at the
top surface; a second region having the second conductivity type,
the second region being formed in the substrate, spaced apart from
the first region and exposed at the top surface; a buried layer in
the substrate below the first and second regions, spaced apart from
the first and second regions and having the second conductivity
type; a body region formed between the first and second regions and
the buried layer, the body region having the first conductivity
type; and a gate positioned between the first and second regions
and above the top surface; a source line terminal electrically
connected to one of the first and second regions; a bit line
terminal electrically connected to the other of the first and
second regions; a word line terminal connected to the gate; a
buried well terminal electrically connected to the buried layer;
and a substrate terminal electrically connected to the substrate
below the buried layer; wherein each memory cell further includes a
first data state which corresponds to a first charge in the body
region, and a second data state which corresponds to a second
charge in the body region; wherein each of the terminals is
controlled to perform operations on each the cell; and wherein the
terminals are controlled to perform a refresh operation by a
non-algorithmic process.
In at least one embodiment, the data state of at least one of the
cells is read by: applying a neutral voltage state to the substrate
terminal, applying a voltage greater than or equal to zero to the
buried well terminal, applying a neutral voltage to the source line
terminal, applying a positive voltage to the bit line terminal and
applying a positive voltage to the word line terminal.
In at least one embodiment, the data state of at least one of the
cells is read by: applying a positive voltage to the substrate
terminal, applying a neutral voltage to the bit line terminal,
applying a positive voltage to the word line terminal and leaving
the source line terminal and the buried well terminal floating.
In at least one embodiment, the first data state is written to at
least one of the cells by: applying a positive voltage to the bit
line terminal, applying a neutral voltage to the source line
terminal, applying a negative voltage to the word line terminal,
applying a positive voltage to the buried well terminal and
applying a neutral voltage to the substrate terminal.
In at least one embodiment, the first data state is written to at
least one of the cells by: applying a positive voltage to the
substrate terminal, applying a neutral voltage to the source line
terminal, applying a positive voltage to the bit line terminal,
applying a positive voltage to the word line terminal and allowing
the buried well terminal to float.
In at least one embodiment, the first data state is written to at
least one of the cells by: applying a neutral voltage to the bit
line terminal, applying a positive voltage to the word line
terminal, applying a positive voltage to the substrate terminal and
allowing the source line terminal and the buried well terminal to
float.
In at least one embodiment, the second data state is written to at
least one of the cells by: applying a negative voltage to the
source line terminal, applying a voltage less than or equal to
about zero to the word line terminal, applying a neutral voltage to
the substrate terminal, applying a voltage greater than or equal to
zero to the buried well terminal, and applying a neutral voltage to
the bit line terminal.
In at least one embodiment, the second data state is written to at
least one of the cells by: applying a positive voltage to the bit
line terminal, applying a positive voltage to the word line
terminal, applying a positive voltage to the substrate terminal,
while allowing the source line terminal and the buried well
terminal to float.
In at least one embodiment, a holding operation is performed on at
least one of the cells by: applying a substantially neutral voltage
to the bit line terminal, applying a neutral or negative voltage to
the word line terminal, and applying a positive voltage to the
substrate terminal, while allowing the source line terminal and the
buried well terminal to float.
A semiconductor memory array is provided, including: a plurality of
semiconductor dynamic random access memory cells arranged in a
matrix of rows and columns, each semiconductor dynamic random
access memory cell including: a substrate being made of a material
having a first conductivity type selected from p-type conductivity
type and n-type conductivity type; a first region having a second
conductivity type selected from the p-type and n-type conductivity
types, the second conductivity type being different from the first
conductivity type; a second region having the second conductivity
type, the second region being spaced apart from the first region; a
buried layer in the substrate below the first and second regions,
spaced apart from the first and second regions and having the
second conductivity type; a body region formed between the first
and second regions and the buried layer, the body region having the
first conductivity type; and a gate positioned between the first
and second regions and adjacent the body region; wherein each
memory cell further includes a first data state which corresponds
to a first charge in the body region, and a second data state which
corresponds to a second charge in the body region; wherein the
substrates of a plurality of the cells are connected to a same
substrate terminal; and wherein data states of the plurality of
cells are maintained by biasing the substrate terminal.
In at least one embodiment, the cells are refreshed by a
non-algorithmic process.
In at least one embodiment, the voltage applied to the substrate
terminal automatically activates each cell of the plurality of
cells that has the first data state to refresh the first data
state, and wherein each cell of the plurality of cells that has the
second data state automatically remains deactivated upon
application of the voltage to the substrate terminal so that each
the cell having the second data state remains in the second data
state.
In at least one embodiment, the substrate terminal is periodically
biased by pulsing the substrate terminal and wherein the data
states of the plurality of cells are refreshed upon each the
pulse.
In at least one embodiment, the substrate terminal is constantly
biased and the plurality of cells constantly maintain the data
states.
In at least one embodiment, the substrate has a top surface, the
first region is formed in the substrate and exposed at the top
surface; wherein the second region is formed in the substrate and
exposed at the top surface; and wherein the gate is positioned
above the top surface.
In at least one embodiment, the first and second regions are formed
in a fin that extends above the buried layer, the gate is provided
on opposite sides of the fin, between the first and second regions,
and the body region is between the first and second regions and
between the gate on opposite sides of the fin.
In at least one embodiment, the gate is additionally provided
adjacent a top surface of the body region.
These and other features of the invention will become apparent to
those persons skilled in the art upon reading the details of the
devices and methods as more fully described below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional, schematic view of a memory cell
according to an embodiment of the present invention.
FIGS. 2A-2B illustrate various voltage states applied to terminals
of a memory cell or plurality of memory cells, to carry out various
functions according to various embodiments of the present
invention.
FIG. 3 illustrates an operating condition for a write state "1"
operation that can be carried out on a memory cell according to an
embodiment of the present invention.
FIG. 4 illustrates an operating condition for a write state "0"
operation that can be carried out on a memory cell according to an
embodiment of the present invention.
FIG. 5 illustrates a holding operation that can be carried out on a
memory cell according to an embodiment of the present
invention.
FIGS. 6-7 illustrate cross-sectional schematic illustrations of
fin-type semiconductor memory cell devices according to embodiments
of the present invention
FIG. 8 illustrate a top view of a fin-type semiconductor memory
cell device according to the embodiment shown in FIG. 6.
FIG. 9 is a schematic diagram showing an example of array
architecture of a plurality memory cells according to an embodiment
of the present invention.
FIG. 10 is a schematic diagram showing an example of array
architecture of a plurality memory cells according to another
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Before the present devices and methods are described, it is to be
understood that this invention is not limited to particular
embodiments described, as such may, of course, vary. It is also to
be understood that the terminology used herein is for the purpose
of describing particular embodiments only, and is not intended to
be limiting, since the scope of the present invention will be
limited only by the appended claims.
Where a range of values is provided, it is understood that each
intervening value, to the tenth of the unit of the lower limit
unless the context clearly dictates otherwise, between the upper
and lower limits of that range is also specifically disclosed. Each
smaller range between any stated value or intervening value in a
stated range and any other stated or intervening value in that
stated range is encompassed within the invention. The upper and
lower limits of these smaller ranges may independently be included
or excluded in the range, and each range where either, neither or
both limits are included in the smaller ranges is also encompassed
within the invention, subject to any specifically excluded limit in
the stated range. Where the stated range includes one or both of
the limits, ranges excluding either or both of those included
limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used
herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
any methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, the preferred methods and materials are now described.
All publications mentioned herein are incorporated herein by
reference to disclose and describe the methods and/or materials in
connection with which the publications are cited.
It must be noted that as used herein and in the appended claims,
the singular forms "a", "an", and "the" include plural referents
unless the context clearly dictates otherwise. Thus, for example,
reference to "a memory cell" includes a plurality of such memory
cells and reference to "the device" includes reference to one or
more devices and equivalents thereof known to those skilled in the
art, and so forth.
The publications discussed herein are provided solely for their
disclosure prior to the filing date of the present application.
Nothing herein is to be construed as an admission that the present
invention is not entitled to antedate such publication by virtue of
prior invention. Further, the dates of publication provided may be
different from the actual publication dates which may need to be
independently confirmed.
Definitions
When a terminal is referred to as being "left floating", this means
that the terminal is not held to any specific voltage, but is
allowed to float to a voltage as driven by other electrical forces
with the circuit that it forms a part of.
The term "refresh" or "refresh operation" refers to a process of
maintaining charge (and the corresponding data) of a memory cell,
typically a dynamic random access memory (DRAM) cell. Periodic
refresh operations of a DRAM cell are required because the stored
charge leaks out over time.
Description
The present invention provides capacitorless DRAM memory cells that
are refreshable by a non-algorithmic process. Alternatively, the
memory cells may be operated to maintain memory states without the
need to refresh the memory states, similar to SRAM memory
cells.
FIG. 1 shows an embodiment of a memory cell 50 according to the
present invention. The cell 50 includes a substrate 12 of a first
conductivity type, such as a p-type conductivity type, for example.
Substrate 12 is typically made of silicon, but may comprise
germanium, silicon germanium, gallium arsenide, carbon nanotubes,
or other semiconductor materials known in the art. The substrate 12
has a surface 14. A first region 16 having a second conductivity
type, such as n-type, for example, is provided in substrate 12 and
is exposed at surface 14. A second region 18 having the second
conductivity type is also provided in substrate 12, and is also
exposed at surface 14. Second region 18 is spaced apart from the
first region 16, as shown. First and second regions 16 and 18 are
formed by an implantation process formed on the material making up
substrate 12, according to any of implantation processes known and
typically used in the art.
A buried layer 22 of the second conductivity type is also provided
in the substrate 12, buried in the substrate 12, as shown. Buried
layer 22 is also formed by an ion implantation process on the
material of substrate 12. A body region 24 of the substrate 12 is
bounded by surface 14, first and second regions 16,18, insulating
layers 26 and buried layer 22. Insulating layers 26 (e.g., shallow
trench isolation (STI)), may be made of silicon oxide, for example.
Insulating layers 26 insulate cell 50 from neighboring cells 50
when multiple cells 50 are joined in an array 80 to make a memory
device. A gate 60 is positioned in between the regions 16 and 18,
and above the surface 14. The gate 60 is insulated from surface 14
by an insulating layer 62. Insulating layer 62 may be made of
silicon oxide and/or other dielectric materials, including high-K
dielectric materials, such as, but not limited to, tantalum
peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or
aluminum oxide. The gate 60 may be made of polysilicon material or
metal gate electrode, such as tungsten, tantalum, titanium and
their nitrides.
Cell 50 further includes word line (WL) terminal 70 electrically
connected to gate 60, source line (SL) terminal 72 electrically
connected to one of regions 16 and 18 (connected to 16 as shown,
but could, alternatively, be connected to 18), bit line (BL)
terminal 74 electrically connected to the other of regions 16 and
18, buried well (BW) terminal 76 electrically connected to buried
layer 22, and substrate terminal 78 electrically connected to
substrate 12 at a location beneath buried layer 22.
FIG. 2 illustrates relative voltages that can be applied to the
terminals of memory cell 50 to perform various operations. For a
read operation, a neutral voltage (i.e., about zero volts) is
applied to the substrate terminal 78, a neutral or positive voltage
(greater than or equal to about zero volts) is applied to the BW
terminal 76, a neutral voltage (about zero volts) is applied to SL
terminal 72, a positive voltage is applied to BL terminal 74, and a
positive voltage is applied to WL terminal 70, with the voltage at
terminal 70 being more positive (higher voltage) that the voltage
applied to terminal 74. If cell 50 is in a state "1" having holes
in the body region 24, then a lower threshold voltage (gate voltage
where the transistor is turned on) is observed compared to the
threshold voltage observed when cell 50 is in a state "0" having no
holes in body region 24. In one particular non-limiting embodiment,
about 0.0 volts is applied to terminal 72, about +0.4 volts is
applied to terminal 74, about +1.2 volts is applied to terminal 70,
about +0.6 volts is applied to terminal 76, and about 0.0 volts is
applied to terminal 78. However, these voltage levels may vary.
Alternatively, a neutral voltage is applied to the substrate
terminal 78, a neutral or positive voltage is applied to the BW
terminal 76, a neutral voltage is applied to SL terminal 72, a
positive voltage is applied to BL terminal 74, and a positive
voltage is applied to WL terminal 70, with the voltage at terminal
74 being more positive (higher voltage) that the voltage applied to
terminal 70. If cell 50 is in a state "1" having holes in the body
region 24, then the parasitic bipolar transistor formed by the SL
terminal 72, floating body 24, and BL terminal 74 will be turned on
and a higher cell current is observed compared to when cell 50 is
in a state "0" having no holes in body region 24. In one particular
non-limiting embodiment, about 0.0 volts is applied to terminal 72,
about +3.0 volts is applied to terminal 74, about +0.5 volts is
applied to terminal 70, about +0.6 volts is applied to terminal 76,
and about 0.0 volts is applied to terminal 78. However, these
voltage levels may vary.
Alternatively, a positive voltage is applied to the substrate
terminal 78, a substantially neutral voltage is applied to BL
terminal 74, and a positive voltage is applied to WL terminal 70.
The SL terminal 72 and the BW terminal 76 are left floating, as
shown in FIG. 2. Cell 50 provides a P1-N2-P3-N4 silicon controlled
rectifier device, with substrate 78 functioning as the P1 region,
buried layer 22 functioning as the N2 region, body region 24
functioning as the P3 region and region 16 or 18 functioning as the
N4 region. In this example, the substrate terminal 78 functions as
the anode and terminal 72 or terminal 74 functions as the cathode,
while body region 24 functions as a p-base to turn on the SCR
device. If cell 50 is in a state "1" having holes in the body
region 24, the silicon controlled rectifier (SCR) device formed by
the substrate, buried well, floating body, and the BL junction will
be turned on and a higher cell current is observed compared to when
cell 50 is in a state "0" having no holes in body region 24. A
positive voltage is applied to WL terminal 70 to select a row in
the memory cell array 80 (e.g., see FIGS. 9-10), while negative
voltage is applied to WL terminal 70 for any unselected rows. The
negative voltage applied reduces the potential of floating body 24
through capacitive coupling in the unselected rows and turns off
the SCR device of each cell 50 in each unselected row. In one
particular non-limiting embodiment, about +0.8 volts is applied to
terminal 78, about +0.5 volts is applied to terminal 70 (for the
selected row), and about 0.0 volts is applied to terminal 74.
However, these voltage levels may vary.
FIG. 3 illustrate a write state "1" operation that can be carried
out on cell 50 according to an embodiment of the invention, by
performing band-to-band tunneling hot hole injection or impact
ionization hot hole injection. To write state "1" using
band-to-band tunneling mechanism, the following voltages are
applied to the terminals: a positive voltage is applied to BL
terminal 74, a neutral voltage is applied to SL terminal 72, a
negative voltage is applied to WL terminal 70, a positive voltage
is applied to BW terminal 76, and a neutral voltage is applied to
the substrate terminal 78. Under these conditions, holes are
injected from BL terminal 74 into the floating body region 24,
leaving the body region 24 positively charged. In one particular
non-limiting embodiment, a charge of about 0.0 volts is applied to
terminal 72, a voltage of about +2.0 volts is applied to terminal
74, a voltage of about -1.2 volts is applied to terminal 70, a
voltage of about +0.6 volts is applied to terminal 76, and about
0.0 volts is applied to terminal 78. However, these voltage levels
may vary.
Alternatively, to write state "1" using impact ionization
mechanism, the following voltages are applied to the terminals: a
positive voltage is applied to BL terminal 74, a neutral voltage is
applied to SL terminal 72, a positive voltage is applied to WL
terminal 70, a positive voltage less than the positive voltage
applied to BL terminal 74 is applied to BW terminal 76, and a
neutral voltage is applied to the substrate terminal 78. Under
these conditions, holes are injected from BL terminal 74 into the
floating body region 24, leaving the body region 24 positively
charged. In one particular non-limiting embodiment, +0.0 volts is
applied to terminal 72, a voltage of about +2.0 volts is applied to
terminal 74, a charge of about +0.5 volts is applied to terminal
70, a charge of about +0.6 volts is applied to terminal 76, and
about 0.0 volts is applied to terminal 78. However, these voltage
levels may vary.
In an alternate write state "1" using impact ionization mechanism,
a positive bias can be applied to substrate terminal 78, a positive
voltage greater than or equal to the positive voltage applied to
substrate terminal 78 is applied to BL terminal 74, a neutral
voltage is applied to SL terminal 72, a positive voltage is applied
to WL terminal 70, while the BW terminal 76 is left floating. The
parasitic silicon controlled rectifier device of the selected cell
is now turned off due to the negative potential between the
substrate terminal 78 and the BL terminal 74. Under these
conditions, electrons will flow near the surface of the transistor,
and generate holes through the impact ionization mechanism. The
holes are subsequently injected into the floating body region 24.
In one particular non-limiting embodiment, about +0.0 volts is
applied to terminal 72, a voltage of about +2.0 volts is applied to
terminal 74, a voltage of about +0.5 volts is applied to terminal
70, and about +0.8 volts is applied to terminal 78, while terminal
76 is left floating. However, these voltage levels may vary, while
maintaining the relative relationships between the charges applied,
as described above.
Alternatively, the silicon controlled rectifier device of cell 50
can be put into a state "1" (i.e., by performing a write "1"
operation) by applying the following bias: a neutral voltage is
applied to BL terminal 74, a positive voltage is applied to WL
terminal 70, and a positive voltage is applied to the substrate
terminal 78, while SL terminal 72 and BW terminal 76 are left
floating. The positive voltage applied to the WL terminal 70 will
increase the potential of the floating body 24 through capacitive
coupling and create a feedback process that turns the SCR device
on. Once the SCR device of cell 50 is in conducting mode (i.e., has
been "turned on") the SCR becomes "latched on" and the voltage
applied to WL terminal 70 can be removed without affecting the "on"
state of the SCR device. In one particular non-limiting embodiment,
a voltage of about 0.0 volts is applied to terminal 74, a voltage
of about +0.5 volts is applied to terminal 70, and about +3.0 volts
is applied to terminal 78. However, these voltage levels may vary,
while maintaining the relative relationships between the voltages
applied, as described above, e.g., the voltage applied to terminal
78 remains greater than the voltage applied to terminal 74.
A write "0" operation of the cell 50 is now described with
reference to FIG. 2B and FIG. 4. To write "0" to cell 50, a
negative bias is applied to SL terminal 72, a neutral voltage is
applied to BL terminal 74, a neutral or negative voltage is applied
to WL terminal 70, a neutral or positive voltage is applied to BW
terminal 76 and a neutral voltage is applied to substrate terminal
78. Under these conditions, the p-n junction (junction between 24
and 18) is forward-biased, evacuating any holes from the floating
body 24. In one particular non-limiting embodiment, about -2.0
volts is applied to terminal 72, about -1.2 volts is applied to
terminal 70, about 0.0 volts is applied to terminal 74, about +0.6
volts is applied to terminal 76 and about 0.0 volts is applied to
terminal 78. However, these voltage levels may vary, while
maintaining the relative relationships between the charges applied,
as described above. Alternatively, the voltages applied to
terminals 72 and 74 may be switched.
Alternatively, a write "0" operation can be performed by putting
the silicon controlled rectifier device into the blocking mode.
This can be performed by applying the following bias: a positive
voltage is applied to BL terminal 74, a positive voltage is applied
to WL terminal 70, and a positive voltage is applied to the
substrate terminal 78, while leaving SL terminal 72 and BW terminal
76 floating. Under these conditions the voltage difference between
anode and cathode, defined by the voltages at substrate terminal 78
and BL terminal 74, will become too small to maintain the SCR
device in conducting mode. As a result, the SCR device of cell 50
will be turned off. In one particular non-limiting embodiment, a
voltage of about +0.8 volts is applied to terminal 74, a voltage of
about +0.5 volts is applied to terminal 70, and about +0.8 volts is
applied to terminal 78. However, these voltage levels may vary,
while maintaining the relative relationships between the charges
applied, as described above.
A holding or standby operation is described with reference to FIGS.
2B and 5. Such holding or standby operation is implemented to
enhance the data retention characteristics of the memory cells 50.
The holding operation can be performed by applying the following
bias: a substantially neutral voltage is applied to BL terminal 74,
a neutral or negative voltage is applied to WL terminal 70, and a
positive voltage is applied to the substrate terminal 78, while
leaving SL terminal 72 and BW terminal 76 floating. Under these
conditions, if memory cell 50 is in memory/data state "1" with
positive voltage in floating body 24, the SCR device of memory cell
50 is turned on, thereby maintaining the state "1" data. Memory
cells in state "0" will remain in blocking mode, since the voltage
in floating body 24 is not substantially positive and therefore
floating body 24 does not turn on the SCR device. Accordingly,
current does not flow through the SCR device and these cells
maintain the state "0" data. In this way, an array of memory cells
50 can be refreshed by periodically applying a positive voltage
pulse through substrate terminal 78. Those memory cells 50 that are
commonly connected to substrate terminal 78 and which have a
positive voltage in body region 24 will be refreshed with a "1"
data state, while those memory cells 50 that are commonly connected
to the substrate terminal 78 and which do not have a positive
voltage in body region 24 will remain in blocking mode, since their
SCR device will not be turned on, and therefore memory state "0"
will be maintained in those cells. In this way, all memory cells 50
commonly connected to the substrate terminal will be
maintained/refreshed to accurately hold their data states. This
process occurs automatically, upon application of voltage to the
substrate terminal 78, in a parallel, non-algorithmic, efficient
process. In one particular non-limiting embodiment, a voltage of
about 0.0 volts is applied to terminal 74, a voltage of about -1.0
volts is applied to terminal 70, and about +0.8 volts is applied to
terminal 78. However, these voltage levels may vary, while
maintaining the relative relationships therebetween. Alternatively,
the voltages applied to terminals 72 and 74 may be reversed.
FIGS. 6-8 show another embodiment of memory cell 50 according to
the present invention. In this embodiment, cell 50 has a fin
structure 52 fabricated on substrate 12, so as to extend from the
surface of the substrate to form a three-dimensional structure,
with fin 52 extending substantially perpendicularly to, and above
the top surface of the substrate 12. Fin structure 52 is conductive
and is built on buried well layer 22. Region 22 is also formed by
an ion implantation process on the material of substrate 12. Buried
well layer 22 insulates the floating substrate region 24, which has
a first conductivity type, from the bulk substrate 12. Fin
structure 52 includes first and second regions 16, 18 having a
second conductivity type. Thus, the floating body region 24 is
bounded by the top surface of the fin 52, the first and second
regions 16, 18 the buried well layer 22, and insulating layers 26
(see insulating layers 26 in FIG. 8). Insulating layers 26 insulate
cell 50 from neighboring cells 50 when multiple cells 50 are joined
to make a memory device. Fin 52 is typically made of silicon, but
may comprise germanium, silicon germanium, gallium arsenide, carbon
nanotubes, or other semiconductor materials known in the art.
Device 50 further includes gates 60 on two opposite sides of the
floating substrate region 24 as shown in FIG. 6. Alternatively,
gates 60 can enclose three sides of the floating substrate region
24 as shown in FIG. 7. Gates 60 are insulated from floating body 24
by insulating layers 62. Gates 60 are positioned between the first
and second regions 16, 18, adjacent to the floating body 24.
Device 50 includes several terminals: word line (WL) terminal 70,
source line (SL) terminal 72, bit line (BL) terminal 74, buried
well (BW) terminal 76 and substrate terminal 78. Terminal 70 is
connected to the gate 60. Terminal 72 is connected to first region
16 and terminal 74 is connected to second region 18. Alternatively,
terminal 72 can be connected to second region 18 and terminal 74
can be connected to first region 16. Terminal 76 is connected to
buried layer 22 and terminal 78 is connected to substrate 12. FIG.
8 illustrates the top view of the memory cell 50 shown in FIG.
6.
FIG. 9 shows an example of array architecture 80 of a plurality of
memory cells 50 arranged in a plurality of rows and columns
according to an embodiment of the present invention. The memory
cells 50 are connected such that within each row, all of the gates
60 are connected by a common word line terminal 70. The first
regions 16 within the same row are also connected by a common
source line 72. Within each column, the second regions 18 are
connected to a common bit line terminal 74. Within each row, all of
the buried layers 22 are connected by a common buried word terminal
76. Likewise, within each row, all of the substrates 12 are
connected by a common substrate terminal 78.
In one embodiment, the buried layer 76 or the substrate 78 can be
segmented (e.g., see FIG. 10) to allow independent control of the
applied bias on the selected portion of the memory array. For
example, the buried layer terminals 76a and 76b are connected
together to form a segment independent of the segment defined by
common buried layer terminals 76m and 76n in FIG. 10. Similarly,
the substrate terminals 78a and 78b can form a segment that can be
biased independently from other segments, for example, the segment
defined by substrate terminals 78m and 78n. This array segmentation
allows one segment of the memory array 80 to perform one operation
(e.g., read), while the other segments perform another operation
(e.g., holding).
From the foregoing it can be seen that with the present invention,
a semiconductor memory with electrically floating body is achieved,
and that this memory can be operated to perform non-algorithmic
refreshment of the data stored in such memory. Additionally, such
restore operations can be performed on the memory cells
automatically, in parallel. The present invention also provides the
capability of maintaining memory states without the need for
periodic refresh operations by application of a constant positive
bias to the substrate terminal. As a result, memory operations can
be performed in an uninterrupted manner. While the foregoing
written description of the invention enables one of ordinary skill
to make and use what is considered presently to be the best mode
thereof, those of ordinary skill will understand and appreciate the
existence of variations, combinations, and equivalents of the
specific embodiment, method, and examples herein. The invention
should therefore not be limited by the above described embodiment,
method, and examples, but by all embodiments and methods within the
scope and spirit of the invention as claimed.
* * * * *